1. Field of the Invention
The present invention generally relates to communication apparatus and, more particularly, is directed to a communication apparatus using a power amplifier of a low-power consumption type.
2. Description of the Prior Art
Recently, in land mobile communication such as a mobile telephone, a portable telephone or the like, the present radio frequency band allocated to the land mobile communication becomes insufficient for radio communication due to increase of subscribers. Then, various new services for the land mobile communication have been decided to be started. These new services are common in that they are each performed by digital communication of the time-division multiplex system using a quasi-microwave band of 0.8 GHz to 2 GHz.
Terminal devices for such a land mobile communication are indispensably required to be miniaturized since portability is a main feature of the devices. Then, integration of electronic parts used in the device has become a most important problem. Further, since the terminal device must be necessarily driven by a battery, it is an important condition to decrease a power source current of the device in order to make a non-charging time period for the battery longer.
Of the power source currents, an amount of current consumed by a power amplifier integrated circuit at a transmission stage is quite larger when compared with the consumed currents at the other portions of the terminal device. Therefore, it has been an important problem to decrease an amount of the current consumed by the power amplifier integrated circuit.
The conventional mobile communication terminal device of such a type is constructed as shown in FIG. 1, for example. The apparatus shown in FIG. 1 is an example of the mobile communication terminal device which is used in the time-division multiplex communication system.
Referring to FIG. 1, a relay 2 is connected between a power source terminal of a power amplifier 4 and one electrode of a power source 3, the other electrode of which is grounded at 5. Since the time-division multiplex system is employed, the relay 2 intermittently makes and breaks the connection between the power source 3, and the power amplifier 4 in accordance with a control signal from a main controller 1. Therefore, the power amplifier 4 of the terminal device is operated only during a time slot which is allotted to the terminal device itself, so that an amount of power source current consumed by the power amplifier 4 is decreased.
However, this conventional mobile communication terminal device has the following two serious defects.
The first defect is that the power source voltage of the power amplifier 4 is intermittently on and off. In this case, since many capacitors are inserted in the power source terminal of the power amplifier 4, it is impossible to turn on and off the power source voltage at a high speed, and so the power amplifier 4 is made on and off during the time slots adjacent its own time slot, respectively. However, since much power source current which is not effective to the power amplifier 4 continues to flow therethrough during the adjacent time slots, there arises a serious defect that the power source current can not be saved.
The second defect is that power consumed by the power amplifier 4 of the terminal device during its own time slot is constant regardless of an amount of the transmission power. An average transmission power is small in such a system as the portable telephone in which transmission power is always changed. However, since the power consumed by the power amplifier 4 is always constant, an average efficiency of the terminal device is degraded. As a consequence, there arises a serious defect that an amount of power source current consumed uselessly increases.